Chapter 8 Precambrian Earth and Life HistoryThe Archean

Chapter 8 Precambrian Earth and Life HistoryThe Archean

Chapter 8 Precambrian Earth and Life HistoryThe Archean Eon Archean Rocks The Beartooth Mountains on the Wyoming and Montana border consists of Archaean-age gneisses,

some of the oldest rocks in the US. Precambrian The Precambrian lasted for more than 4 billion years! This large time span is difficult for humans to comprehend Suppose that a 24-hour clock represented all 4.6 billion years of geologic time then the Precambrian would be slightly more than 21 hours long,

constituting about 88% of all geologic time Precambrian Time Span 88% of geologic time Precambrian The term Precambrian is informal but widely used, referring to both time and rocks The Precambrian includes time from Earths origin 4.6 billion years ago to the beginning of the Phanerozoic Eon

542 million years ago It encompasses all rocks below the Cambrian system No rocks are known for the first 600 million years of geologic time The oldest known rocks on Earth are 4.0 billion years old Rocks Difficult to Interpret The earliest record of geologic time preserved in rocks is difficult to interpret because many Precambrian rocks have been

altered by metamorphism complexly deformed buried deep beneath younger rocks fossils are rare, and the few fossils present are not of any use in biostratigraphy Subdivisions of the Precambrian have been difficult to establish

Two eons for the Precambrian are the Archean and Proterozoic which are based on absolute ages Eons of the Precambrian Eoarchean refers to all time from Earths origin to the Paleoarchean 3.6 billion years ago Earths oldest body of rocks the Acasta Gneiss in Canada is about 4.0 billion years old

We have no geologic record for much of the Archaen Precambrian eons have no stratotypes unlike the Cambrian Period, for example What Happened During the Eoarchean? Although no rocks of Eoarchean age are present on Earth, except for meteorites, we do know some events that took place then Earth accreted from planetesimals

and differentiated into a core and mantle and at least some crust was present Earth was bombarded by meteorites Volcanic activity was ubiquitous An atmosphere formed, quite different from todays Oceans began to accumulate Hot, Barren, Waterless Early Earth about 4.6 billion years ago Shortly after accretion, Earth was

a rapidly rotating, hot, barren, waterless planet bombarded by meteorites and comets with no continents, intense cosmic radiation and widespread volcanism Oldest Rocks Continental crust was present by 4.0 billion years ago Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.4 billion years old so source rocks at least that old existed

The Eoarchean Earth probably rotated in as little as 10 hours and the Earth was closer to the Moon By 4.4 billion years ago, the Earth cooled sufficiently for surface waters to accumulate Eoarchean Crust Early crust formed as upwelling mantle currents of mafic magma, and numerous subduction zones developed to form the first island arcs

Eoarchean continental crust may have formed by collisions between island arcs as silica-rich materials were metamorphosed. Larger groups of merged island arcs protocontinents grew faster by accretion along their margins Origin of Continental Crust Andesitic island arcs form by subduction

and partial melting of oceanic crust The island arc collides with another Continental Foundations Continents consist of rocks with composition similar to that of granite Continental crust is thicker

and less dense than oceanic crust which is made up of basalt and gabbro Precambrian shields consist of vast areas of exposed ancient rocks and are found on all continents Outward from the shields are broad platforms of buried Precambrian rocks that underlie much of each continent Cratons A shield and its platform make up a craton, a continents ancient nucleus

Along the margins of cratons, more continental crust was added as the continents took their present sizes and shapes Both Archean and Proterozoic rocks are present in cratons and show evidence of episodes of deformation accompanied by igneous activity, metamorphism, and mountain building Cratons have experienced little deformation since the Precambrian

Distribution of Precambrian Rocks Areas of exposed Precambrian rocks constitute the shields Platforms consist of buried Precambrian rocks Shields and adjoining platforms make up cratons Canadian Shield

The exposed part of the craton in North America is the Canadian shield which occupies most of northeastern Canada a large part of Greenland parts of the Lake Superior region in Minnesota, Wisconsin, and Michigan and the Adirondack Mountains of New York Its topography is subdued, with numerous lakes and exposed Archean and Proterozoic rocks thinly covered in places by Pleistocene glacial deposits

Evolution of North America North America evolved by the amalgamation of Archean cratons that served as a nucleus around which younger continental crust was added. North American Craton Drilling and geophysical evidence indicate that Precambrian rocks underlie much of North America, exposed only in places by deep erosion or uplift Archean Rocks Only 22% of Earths exposed Precambrian crust is

Archean The most common Archean rock associations are granite-gneiss complexes Other rocks range from peridotite to various sedimentary rocks all of which have been metamorphosed Greenstone belts are subordinate in quantity, account for only 10% of Archean rocks but are important in unraveling Archean tectonic events Archean Rocks

Outcrop of Archean gneiss cut by a granite dike from a granite-gneiss complex in Ontario, Canada Archean Rocks Shell Creek in the Bighorn Mountains of Wyoming has cut a gorge into this 2.9 billion year old granite Greenstone Belts A greenstone belt has 3 major rock units volcanic rocks are most common in the lower and middle units the upper units are mostly sedimentary

The belts typically have synclinal structure Most were intruded by granitic magma and cut by thrust faults Low-grade metamorphism makes many of the igneous rocks green Because they contain chlorite, actinolite, and epidote Greenstone Belts and GraniteGneiss Complexes Two adjacent greenstone belts showing synclinal

structure They are underlain by granite-gneiss complexes and intruded by granite Greenstone Belt Volcanics Pillow lavas in greenstone belts indicate that much of the volcanism was subaqueous Pyroclastic

materials probably erupted where large volcanic centers built above sea level Pillow lavas in Ispheming greenstone belt at Marquette, Michigan Ultramafic Lava Flows The most interesting rocks in greenstone belts are komatiites,

cooled from ultramafic lava flows Ultramafic magma (< 45% silica) requires near surface magma temperatures of more than 1600C 250C hotter than any recent flows During Earths early history, radiogenic heating was greater and the mantle was as much as 300 C hotter than it is now This allowed ultramafic magma to reach the surface

Ultramafic Lava Flows As Earths production of radiogenic heat decreased, the mantle cooled and ultramafic flows no longer occurred They are rare in rocks younger than Archean and none occur now Sedimentary Rocks of Greenstone Belts Sedimentary rocks are found throughout the greenstone belts

although they predominate in the upper unit Many of these rocks are successions of graywacke sandstone with abundant clay and rock fragments and argillite slightly metamorphosed mudrock Sedimentary Rocks of Greenstone Belts Small-scale cross-bedding and graded bedding indicate an origin as turbidity current deposits

Other sedimentary rocks are present, but not abundant sandstone, conglomerate, chert, carbonates Iron-rich rocks, banded iron formations, are more typical of Proterozoic deposits Canadian Greenstone Belts In North America, most greenstone belts

(dark green) occur in the Superior and Slave cratons of the Canadian shield Evolution of Greenstone Belts Greenstone belts formed in several tectonic settings Models for the formation of greenstone belts involve Archean plate movement In one model, greenstone

belts formed in back-arc marginal basins Evolution of Greenstone Belts According to this model, There was an early stage of extension as the backarc marginal basin formed volcanism and sediment deposition followed Evolution of Greenstone Belts Then during closure,

the rocks were compressed, metamorphosed, and intruded by granitic magma The Sea of Japan is a modern example of a back-arc basin Another Model In another model accepted by some geologists,

greenstone belts formed over rising mantle plumes in intracontinental rifts As the plume rises beneath sialic crust it spreads and generates tensional forces The mantle plume is the source of the volcanic rocks in the lower and middle units of the greenstone belt and erosion of volcanic rocks and flanks for the rift supply the sediment to the upper unit An episode of subsidence, deformation, metamorphism and plutonism followed

Greenstone BeltsIntracontinental Rift Model Ascending mantle plume causes rifting and volcanism Greenstone BeltsIntracontinental Rift Model Erosion of the rift flanks accounts for sediments

Greenstone BeltsIntracontinental Rift Model Closure of rift causes compression and deformation Archean Plate Tectonics Plate tectonic activity has operated since the Paleoproterozoic or earlier Most geologists are convinced that some kind of plate tectonic activity

took place during the Archean as well but it differed in detail from today Plates must have moved faster with more residual heat from Earths origin and more radiogenic heat, and magma was generated more rapidly Archean Plate Tectonics As a result of the rapid movement of plates, continents grew more rapidly along their margins a process called continental accretion as plates collided with island arcs and other plates

Also, ultramafic extrusive igneous rocks, komatiites, were more common Archean World Differences The Archean world was markedly different than later We have little evidence of Archean rocks deposited on broad, passive continental margins Deformation belts between colliding

cratons indicate that Archean plate tectonics was active but associations of passive continental margin sediments are widespread in Proterozoic terrains but the ophiolites so typical of younger convergent plate boundaries are rare,

although Neoarchean ophiolites are known The Origin of Cratons Certainly several small cratons existed during the Archean and grew by accretion along their margins They amalgamated into a larger unit during the Proterozoic By the end of the Archean, 30-40% of the present volume of continental crust existed

Archean crust probably evolved similarly to the evolution of the southern Superior craton of Canada Southern Superior Craton Evolution Geologic map Plate tectonic model for evolution of the southern Superior craton North-south cross section

Greenstone belts (dark green) Granite-gneiss complexes (light green Canadian Shield Deformation of the southern Superior craton was part of a more extensive orogenic episode during the Mesoarchean and Neoarchean that formed the Superior and Slave cratons and some Archean rocks in Wyoming, Montana, and the Mississippi River Valley

By the time this Archean event ended several cratons had formed that are found in the older parts of the Canadian shield Atmosphere and Hydrosphere Earths early atmosphere and hydrosphere were quite different than they are now They also played an important role in the development of the biosphere Todays atmosphere is mostly nitrogen (N2)

abundant free oxygen (O2), or oxygen not combined with other elements such as in carbon dioxide (CO2) water vapor (H2O) small amounts of other gases, like ozone (O3) which is common enough in the upper atmosphere to block most of the Suns ultraviolet radiation Present-day Atmosphere Composition Nonvariable gases

Nitrogen N2 78.08% Variable gases Water vapor H2O Oxygen Ozone O2 20.95 Argon Ar 0.93 Neon Ne

0.002 Others 0.001 in percentage by volume Carbon dioxide CO2 0.1 to 4.0 0.038 O3 0.000006

Other gases Particulates Trace normally trace Earths Very Early Atmosphere Earths very early atmosphere was probably composed of hydrogen and helium, the most abundant gases in the universe

If so, it would have quickly been lost into space because Earths gravity is insufficient to retain them because Earth had no magnetic field until its core formed (magnetosphere) Without a magnetic field, the solar wind would have swept away any atmospheric gases Outgassing Once a magnetosphere was present Atmosphere began

accumulating as a result of outgassing released during volcanism Water vapor is the most common volcanic gas today but volcanoes also emit carbon dioxide, sulfur dioxide, carbon monoxide, sulfur, hydrogen, chlorine, and nitrogen Archean Atmosphere

Archean volcanoes probably emitted the same gases, and thus an atmosphere developed but one lacking free oxygen and an ozone layer It was rich in carbon dioxide, and gases reacting in this early atmosphere probably formed ammonia (NH3) methane (CH4) This early atmosphere persisted throughout the Archean

Evidence for an Oxygen-Free Atmosphere The atmosphere was chemically reducing rather than an oxidizing one Some of the evidence for this conclusion comes from detrital deposits containing minerals that oxidize rapidly in the presence of oxygen pyrite (FeS2) uraninite (UO2) But oxidized iron becomes increasingly common in Proterozoic rocks

indicating that at least some free oxygen was present then Introduction of Free Oxygen Two processes account for introducing free oxygen into the atmosphere, one or both of which began during the Eoarchean. 1. Photochemical dissociation involves ultraviolet radiation in the upper atmosphere The radiation disrupts water molecules and releases their oxygen and hydrogen This could account for 2% of present-day oxygen but with 2% oxygen, ozone forms, creating a barrier

against ultraviolet radiation 2. More important were the activities of organisms that practiced photosynthesis Photosynthesis Photosynthesis is a metabolic process in which carbon dioxide and water to make organic molecules and oxygen is released as a waste product CO2 + H2O ==> organic compounds + O2 Even with photochemical dissociation and photosynthesis,

probably no more than 1% of the free oxygen level of today was present by the end of the Archean Oxygen Forming Processes Photochemical dissociation and photosynthesis added free oxygen to the atmosphere Once free oxygen was present an ozone layer formed and blocked incoming

ultraviolet radiation Earths Surface Waters Outgassing was responsible for the early atmosphere and also for some of Earths surface water the hydrosphere most of which is in the oceans more than 97% Another source of our surface water was meteorites and icy comets

Numerous erupting volcanoes, and an early episode of intense meteorite and comet bombardment accounted for rapid rate of surface water accumulation Ocean Water Volcanoes still erupt and release water vapor Is the volume of ocean water still increasing? Perhaps it is, but if so, the rate has decreased considerably because the amount of heat needed to generate magma has diminished

Decreasing Heat Ratio of radiogenic heat production in the past to the present The width of the colored band indicates variations in ratios from different models With less heat

outgassing decreased Heat production 4 billion years ago was 3 to 6 times as great as it is now First Organisms Today, Earths biosphere consists of millions of species of archea, bacteria, fungi, protists, plants, and animals, whereas only bacteria and archea are found in Archean

rocks We have fossils from Archean rocks 3.5 billion years old Chemical evidence in rocks in Greenland that are 3.8 billion years old convince some investigators that organisms were present then What Is Life? Minimally, a living organism must reproduce and practice some kind of metabolism

Reproduction ensures the long-term survival of a group of organisms whereas metabolism maintains the organism The distinction between living and nonliving things is not always easy Are viruses living? When in a host cell they behave like living organisms but outside they neither reproduce nor metabolize

What Is Life? Comparatively simple organic (carbon based) molecules known as microspheres form spontaneously can even grow and divide in a somewhat organism-like fashion but their processes are more like random chemical reactions, so they are not living How Did Life First Originate? To originate by natural processes,

from non-living matter (abiogenesis), life must have passed through a prebiotic stages in which it showed signs of living but was not truly living The origin of life has 2 requirements a source of appropriate elements for organic molecules energy sources to promote chemical reactions Elements of Life All organisms are composed mostly of carbon (C) hydrogen (H) nitrogen (N)

oxygen (O) all of which were present in Earths early atmosphere as carbon dioxide (CO2) water vapor (H2O) nitrogen (N2) and possibly methane (CH4) and ammonia (NH3) Basic Building Blocks of Life Energy from Lightning, volcanism, and ultraviolet radiation

probably promoted chemical reactions during which C, H, N, and O combined to form monomers such as amino acids Monomers are the basic building blocks of more complex organic molecules Experiment on the Origin of Life Is it plausible that monomers originated in the manner postulated? Experimental evidence indicates that it is

During the late 1950s Stanley Miller synthesized several amino acids by circulating gases approximating the early atmosphere in a closed glass vessel Experiment on the Origin of Life This mixture was subjected to an electric spark to simulate lightning

In a few days it became cloudy Analysis showed that several amino acids typical of organisms had formed Since then, scientists have synthesized all 20 amino acids

found in organisms Polymerization The molecules of organisms are polymers such as proteins and nucleic acids RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) consisting of monomers linked together in a specific sequence How did polymerization take place? Water usually causes depolymerization, however, researchers synthesized molecules

known as proteinoids or thermal proteins some of which consist of more than 200 linked amino acids when heating dehydrated concentrated amino acids Proteinoids These concentrated amino acids spontaneously polymerized to form proteinoids Perhaps similar conditions for polymerization existed on early Earth, but the proteinoids needed to be protected by an outer membrane or they would break down

Experiments show that proteinoids spontaneously aggregate into microspheres which are bounded by cell-like membranes and grow and divide much as bacteria do Proteinoid Microspheres Proteinoid microspheres produced in experiments Proteinoids grow and divide much as

bacteria do Protobionts These proteinoid molecules can be referred to as protobionts that are intermediate between inorganic chemical compounds and living organisms Monomer and Proteinoid Soup The origin-of-life experiments are interesting, but what is their relationship to early Earth? Monomers likely formed continuously and by

the billions and accumulated in the early oceans into a hot, dilute soup The amino acids in the soup might have washed up onto a beach or perhaps cinder cones where they were concentrated by evaporation and polymerized by heat The polymers then washed back into the ocean where they reacted further Next Critical Step Not much is known about the next critical step

in the origin of life the development of a reproductive mechanism The microspheres divide and may represent a protoliving system but in todays cells, nucleic acids, either RNA or DNA are necessary for reproduction The problem is that nucleic acids cannot replicate without protein enzymes, and the appropriate enzymes cannot be made without nucleic acids,

or so it seemed until fairly recently RNA World? Now we know that small RNA molecules can replicate without the aid of protein enzymes Thus, the first replicating systems may have been RNA molecules Some researchers propose an early RNA world in which these molecules were intermediate between inorganic chemical compounds and the DNA-based molecules of organisms

How RNA was naturally synthesized remains an unsolved problem Much Remains to Be Learned Scientists agree on some basic requirements for the origin of life, but the exact steps involved and significance of results are debated Many researchers believe that the earliest organic molecules were synthesized from atmospheric gases but some scientist suggest that life arose instead

near hydrothermal vents on the seafloor Submarine Hydrothermal Vents Seawater seeps into the crust near spreading ridges, becomes heated, rises and discharges Black smokers Discharge water saturated with dissolved minerals Life may have formed near these in the past

Submarine Hydrothermal Vents Several minerals containing zinc, copper, and iron precipitate around them Communities of organisms previously unknown to science, are supported here. Necessary elements, sulfur, and phosphorus are present in seawater Polymerization can take place on surface of clay minerals Protocells were deposited on the ocean floor

Oldest Known Organisms The first organisms were archaea and bacteria both of which consist of prokaryotic cells, cells that lack an internal, membrane-bounded nucleus and other structures Prior to the 1950s, scientists assumed that life must have had a long early history but the fossil record offered little to support this idea The Precambrian, once called Azoic (without life), seemed devoid of life

Oldest Know Organisms Charles Walcott (early 1900s) described structures from the Paleoproterozoic Gunflint Iron Formation of Ontario, Canada that he proposed represented reefs constructed by algae Now called stromatolites, not until 1954 were they shown to be products of organic

activity Present-day stromatolites (Shark Bay, Australia) Stromatolites Different types of stromatolites include irregular mats, columns, and columns linked by mats Stromatolites Present-day stromatolites form and grow as sediment grains are trapped on sticky mats of photosynthesizing cyanobacteria although now they are restricted

to environments where snails cannot live The oldest known undisputed stromatolites are found in rocks in South Africa that are 3.0 billion years old but probable ones are also known from the Warrawoona Group in Australia which is 3.3 to 3.5 billion years old Other Evidence of Early Life Chemical evidence in rocks 3.85 billion years old in Greenland indicate life was perhaps present then The oldest known cyanobacteria

were photosynthesizing organisms but photosynthesis is a complex metabolic process A simpler type of metabolism must have preceded it No fossils are known of these earliest organisms Earliest Organisms The earliest organisms must have resembled tiny anaerobic bacteria meaning they required no oxygen They must have totally depended

on an external source of nutrients that is, they were heterotrophic as opposed to autotrophic organisms that make their own nutrients, as in photosynthesis They all had prokaryotic cells Earliest Organisms The earliest organisms, then, were anaerobic, heterotrophic prokaryotes Their nutrient source was most likely adenosine triphosphate (ATP) from their environment

which was used to drive the energy-requiring reactions in cells ATP can easily be synthesized from simple gases and phosphate so it was available in the early Earth environment Fermentation Obtaining ATP from the surroundings could not have persisted for long because more and more cells competed for the same resources

The first organisms to develop a more sophisticated metabolism probably used fermentation to meet their energy needs Fermentation is an anaerobic process in which molecules such as sugars are split releasing carbon dioxide, alcohol, and energy Photosynthesis A very important biological event occurring in the Archean was the development of

the autotrophic process of photosynthesis This may have happened as much as 3.5 billion years ago These prokaryotic cells were still anaerobic, but as autotrophs they were no longer dependent on preformed organic molecules as a source of nutrients Fossil Prokaryotes Photomicrographs from western Australias

3.3- to 3.5-billion-year-old Warrawoona Group, with schematic restoration shown at the right of each Archean Mineral Resources A variety of mineral deposits are of Archean-age but gold is the most commonly associated, although it is also found in Proterozoic and Phanerozoic rocks This soft yellow metal is prized for jewelry, but it is or has been used as a monetary standard, in glass making, electric circuitry, and chemical industry About half the worlds gold since 1886

has come from Archean and Proterozoic rocks in South Africa Gold mines also exist in Archean rocks of the Superior craton in Canada Archean Sulfide Deposits Archean sulfide deposits of zinc, copper and nickel occur in Australia, Zimbabwe, and in the Abitibi greenstone belt

in Ontario, Canada Some, at least, formed as mineral deposits next to hydrothermal vents on the seafloor, much as they do now around black smokers Chrome About 1/4 of Earths chrome reserves are in Archean rocks, especially in Zimbabwe These ore deposits are found in the volcanic units of greenstone belts where they appear to have formed when crystals settled and became concentrated

in the lower parts of plutons such as mafic and ultramafic sills Chrome is needed in the steel industry The United States has very few chrome deposits so must import most of what it uses Chrome and Platinum One chrome deposit in the United States is in the Stillwater Complex in Montana Low-grade ores were mined there during war times, but they were simply stockpiled

and never refined for chrome These rocks also contain platinum, a precious metal, that is used in the automotive industry in catalytic converters in the chemical industry for cancer chemotherapy Iron Banded Iron formations are sedimentary rocks consisting of alternating layers of silica (chert) and iron minerals About 6% of the worlds

banded iron formations were deposited during the Archean Eon Although Archean iron ores are mined in some areas they are neither as thick nor as extensive as those of the Proterozoic Eon, which constitute the worlds major source of iron Pegmatites Pegmatites are very coarsely crystalline igneous rocks, commonly associated with granite plutons

Some Archean pegmatites, such in the Herb Lake district in Manitoba, Canada, and Rhodesian Province in Africa, contain valuable minerals In addition to minerals of gem quality, Archean pegmatites contain minerals mined for lithium, beryllium, rubidium, and cesium Summary Precambrian encompasses all geologic time from Earths origin to the beginning of the Phanerozoic Eon The term also refers to all rocks

that lie stratigraphically below Cambrian rocks The Precambrian is divided into two eons the Archean and the Proterozoic, which are further subdivided Rocks from the latter part of the Eoarchean indicate crust must have existed, but very little of it has been preserved Summary All continents have an ancient stable nucleus or craton made up of an exposed shield

and a buried platform The exposed part of the North American craton is the Canadian shield, and is make up of smaller units delineated by their ages and structural trends Archean greenstone belts are linear, syncline-like bodies found within much more extensive granite-gneiss complexes Summary Greenstone belts typically consist of

two lower units dominated by igneous rocks and an upper unit of mostly sedimentary rocks They probably formed in back-arc basins and in intracontinental rifts Many geologists are convinced some type of Archean plate tectonics occurred, but plates probably moved faster and igneous activity was more common because Earth had more radiogenic heat Summary The early atmosphere and hydrosphere

formed as a result of outgassing, but this atmosphere lacked free oxygen and contained abundant water vapor and carbon dioxide Models for the origin of life by natural processes require an oxygen deficient atmosphere, the necessary elements for organic molecules, and energy to promote the synthesis of organic molecules Summary The first naturally formed organic molecules were probably monomers,

such as amino acids, that linked together to form more complex polymers such as proteins RNA molecules may have been the first molecules capable of self-replication However, how a reproductive mechanism evolved is not known Summary The only known Archean fossils are of single-celled, prokaryotic bacteria or cyanobacteria but other chemical evidence may indicate presence

of archaea Stromatolites formed by photosynthesizing bacteria are found in rocks as much as 3.5 billion years old Archean mineral resources include gold, chrome, zinc, copper, and nickel

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